1. Field of the Invention
The present invention relates to a developing device of an image forming apparatus for forming an image using the electrophotographic system and, more specifically, to a developing device of an image forming apparatus such as a copying machine, a printer and a facsimile machine.
2. Description of Related Art
Conventionally, for example, in an image forming apparatus such as a copying machine and a laser beam printer using the electrophotographic system, an electrostatic latent image is formed on a surface of an electrophotographic photosensitive member (photosensitive drum), for example, of a cylindrical shape functioning as an image bearing member, the electrostatic latent image is developed by a developing device and the developed image is transferred to a recording material and then fixed thereon by a fixing device to form an image on the recording material.
FIG. 12 shows a schematic structure of a copying machine as an example of a conventional image forming apparatus of the electrophotographic system. In such an image forming apparatus, an image of an original is read by an image reading portion 8, exposure is applied to a surface of a photosensitive drum 1 functioning as an image bearing member from an image writing portion 9 according to an instruction from a controller (not shown) based on data of the read image to form an electrostatic latent image on the photosensitive drum 1. The surface of the photosensitive drum 1 is uniformly charged at a predetermined potential by a charging device 2 before the exposure by the image writing portion 9. Then, a laser beam or the like is irradiated from the image writing portion 9 onto the uniformly charged photosensitive drum 1, whereby an electrostatic latent image is formed on the photosensitive drum 1.
The electrostatic latent image formed on the photosensitive drum 1 is developed as a so-called toner image by a developing device 3 using developer. Thereafter, the developed toner image is carried to a portion opposed to a transfer device 4 (transfer portion) by the rotation of the photosensitive drum 1.
A recording material P such as a recording sheet is fed one by one from a recording material containing cassette 12 by a pickup roller 13 in response to the carrying of the toner image on the photosensitive drum 1 and, at the same time, is transported to a part where the photosensitive drum 1 and the transfer device 4 oppose each other while timing being taken by a registration roller pair 14. Then, when the recording material P passes the part where the photosensitive drum 1 and the transfer device 4 oppose each other, the toner image on the photosensitive drum 1 is transferred onto the recording material P by an action of the transfer device 4.
The recording material P having the toner image transferred thereon is transported to a fixing device 7 by a predetermined transport device 6. The fixing device 7 is provided with a fixing roller pair consisting of a fixing roller 7a and a pressure roller 7c. The recording material P is pressurized by the fixing roller pair 7 and, at the same time, heated by a heater 7b provided in the fixing roller 7a. Thus, unfixed toner on the recording material P is fused and fixed on the recording material P.
Thereafter, the recording material P having the toner image fixed thereon is delivered to a tray 15 provided outside an apparatus main body by a transport roller pair 10, a delivery roller pair 11 and so on.
On the other hand, the surface of the photosensitive drum 1 after the toner image is transferred to the recording material P is subject to removal of transfer residual toner or the like by a cleaner 5 and serves image formation repeatedly. Thus, a series of image forming processes is completed.
An electrostatic latent image such as the one described above that is formed on, for example, the photosensitive drum 1 functioning as an image bearing member by a well-known electrostatic latent image technique is developed by the developing device 3. The developing device 3 is generally provided with a rotatable cylindrical developing sleeve 31 functioning as a developer bearing member in a position opposed to the photosensitive drum 1. That is, the developing device 3 is provided with a developing container 32 for containing developer and defining a developing device main body. A position of the developing container 32 opposed to the photosensitive drum 1 opens along a longitudinal direction of the photosensitive drum 1 (a direction perpendicular to a transport direction of the recording material P). The developing sleeve 31 is positioned in the opening portion and supported rotatably with respect to the developer container 32. The developer in the developing device 3 forms a thin layer of a uniform thickness to be borne on the surface of the developing sleeve 31 and is fed to a developing area where the photosensitive drum 1 and the developing sleeve 31 oppose each other by the rotation of the developing sleeve 31. Then, in the developing area, the toner on the developing sleeve 31 moves to the surface of the photosensitive drum 1, whereby an electrostatic latent image is developed.
FIG. 13 shows a schematic sectional view of an example of a conventional developing device. As illustrated, the photosensitive drum 1 and the developing sleeve 31 are arranged while maintaining a microscopic gap (hereinafter referred to as xe2x80x9cSD gapxe2x80x9d) between them constant. Thus, predetermined development can be performed.
The developer is fed to the developing sleeve 31 by developer agitating and feeding members (agitating members) 37 and 38 provided inside the developing container 32.
FIG. 14 shows an arrangement of the developing sleeve 31 in its axial direction (longitudinal direction). As illustrated, the SD gap can be guaranteed by providing two spacer rollers (abutting rollers) 20, whose outer circumferences abut against the outer circumference of the photosensitive drum 1, functioning as spacer members at both end portions in the longitudinal direction of the developing sleeve 31. The two spacer rollers 20 provided in both ends of the developing sleeve 31 are substantially circular in their cross sections and are arranged such that their rotational centers are concentric with the rotational center of the developing sleeve 31. An outer diameter of the spacer rollers 20 is formed larger than an external diameter of the developing sleeve 31 by a size of the SD gap. In addition, the developing container 32 is pressurized by pressurizing means such as pressurizing springs 51, whereby the spacer rollers 20 are always brought into contact with the outer circumference of the photosensitive drum 1. According to such a structure, the photosensitive drum 1 and the developing sleeve 31 are arranged while keeping the predetermined SD gap.
In general, the spacer rollers 20 are provided with ball bearings (hereinafter referred to simply as xe2x80x9cbearingsxe2x80x9d) 20a inside the spacer rollers 20. Thus, the outer circumferences of the spacer rollers 20 rotate following the rotation of the photosensitive drum 1, and the inner circumferences of the bearings 20a provided inside the spacer rollers 20 is rotated by the rotation of the developing sleeve 31.
Parts of the spacer rollers 20 abutting against the photosensitive drum 1 are formed of a material that is excellent in slidability, hard to be worn and unlikely to scratch the photosensitive drum 1. In general, a POM-based resin or an ultrahigh molecular weight polyethylene resin is used as the material.
As shown in FIG. 14, the developing sleeve 31 is supported at its both ends rotatably with respect to the developing container 32 of the developing device 3 via the bearings 21. In the illustrated example, the developing sleeve 31 is supported with respect to the developing container 32 on the inner side inside positions in which the spacer rollers 20 are provided on the rotary shafts (both end shaft portions) 31a provided at both the ends of the developing sleeve 31.
Moreover, a driving gear 23 to which a driving force is transmitted from driving force transmitting means (not shown) is provided on one end side of the developing sleeve 31. A rotational driving force is given to the developing sleeve 31 by the driving gear 23.
The developing device having the one developing sleeve 31 as described above generally increases a peripheral speed of the developing sleeve 31 to rotate at approximately 150 to 200% of a peripheral speed of the photosensitive drum 1 to perform development.
In order to cope with the increased process speed of image formation (copy speed-up), it is necessary to further increase the peripheral speed of the developing sleeve 31. If the peripheral speed of the developing sleeve 31 is insufficient, a developer is in short supply and an image density decreases.
However, the developing device 3 including the one developing sleeve 31 has the following problems for coping with the speed-up.
That is, fusion bond of a developer due to temperature rising of the developing sleeve 31 occurs by the increased peripheral speed of the developing sleeve 31. Thus, it is likely that a rotational torque of the developing sleeve 31 increases and, moreover, the rotation of the developing sleeve 31 is hindered. In addition, since friction of the developer increases, deterioration of the developer occurs, which is likely to result in image defects.
The inventor of the present invention earnestly repeated examinations in order to solve the above-described problems and, as a result, found that the above-described problems can be solved by providing a plurality of developing sleeves in a developing device to have a multi-stage developing device.
On the other hand, as a result of the examinations by the inventor of the present invention, it was found that the following problems are likely to occur in the multi-stage developing device.
That is, in a developing device with a structure provided with two or more developing sleeves, spacer rollers for guaranteeing an SD gap abut against a surface of a photosensitive drum at four or more parts. However, in such a structure with four spacer rollers, it is likely that the spacer rollers first abut against the surface of the photosensitive drum at three parts and the remaining one spacer roller does not abut against the surface of the photosensitive drum but floats.
It is possible to increase a pressurizing force to the photosensitive drum of the developing container such that the spacer rollers abut against the photosensitive drum at four parts. However, the developing container is deformed by increasing the pressurizing force in this way and, when the spacer rollers are caused to abut against the photosensitive drum in a strained state (a state in which stress is applied to it), vibration that causes image deterioration occurs by the stress applied to the developing container. Moreover, it is likely that such a strained state results in breakage of the developing container.
The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide a developing device that is capable of realizing speed-up of image formation and, at the same time, realizing its miniaturization by performing development using a first developer bearing member and a second developer bearing member.
It is another object of the present invention to provide a developing device that is capable of guaranteeing a distance between an image bearing member and the second developer bearing member while guaranteeing a distance between the first developer bearing member and the second developer bearing member.
Further objects of the present invention will be apparent by reading the following detailed description with reference to the attached drawings.